Waveguide support system comprising a liquid-filled duct

ABSTRACT

Sagging, and, therefore, attenuation, is reduced in a waveguide by supporting the waveguide in a liquid-filled duct. The waveguide may be partially or totally immersed in the liquid. If totally immersed the liquid may be combination of two immiscible fluids. The waveguide support system is useful in long distance signal transmission systems.

A United States Patent 11 l 3,909,756

Ritchie 1 Sept. 30, 1975 [54] WAVEGUIDE SUPPORT SYSTEM 3.()()7,|22 lO/lQfil Geyling 333/95 A 3.040.760 Q/l962 MlCkS l38/l ll X I 3,7l3 275 l/l973 Hyatt l38/lll X (75] Inventor: l i m rr h r g 3 748 6()6 7 1973 Kaufman ct 333/98 M x England 3,818.]16 6/l974 Kuljian v. l74/l5 C [73] Assigncc: The P024 Office. London. England 22 Filed; June 14 1974 Primary E\'a/nincrPaul L. Gensler Attorney, Agent, or FirmHall & Houghton [2]] Appl. No.: 479,415

130] Foreign Application Priority Data V l ABSTRACT June 19. 1973 United Kingdom 29005/73 v S a gging, andjthcrcfore, attenuation, is reduced in a U-S. R; l l; gui pp g the waveguide in a 174/15 C; 333/98 R; 333/98 M filled duct. The waveguide may be partially or totally 1!}!- (JF immersed in the If totally immcrscd the [58] of Seal-cl; 333/98 95 R; may be combination of two immiscible fluids. The 106408 1114 174/15 C waveguide support system is useful in long distance signal transmission systems. {56] References Cited UNITED STATES PATENTS 8 Claims, 5 Drawing Figures 1564.007 X/l95l Hochgral 333/98 R y 1 i 2 I .i J 4. A

US. Patent Sept. 30,1975 Sheet 1 of3 US. Patent Sept. 30,1975 Sheet 2 on 3,909,756

U.S. Patent Sept. 30,1975 Sheet 3 of3 3,909,756

WAVEGUIDE SUPPORT SYSTEM COMPRISING A LIQUID-FILLED DUCT This invention relates to a method and system of installing and supporting waveguides of the type which are used in long distance signal transmission systems.

It has been proposed to provide support members for waveguides of the above type, the support members being disposed along the length of the waveguide to support it in a duct or channel. This method of support has the effect of significantly increasing the attenuation of waves transmitted along the waveguide, because the waveguide tends to sag between the support members, introducing additional distributed impedance along the waveguide channel. This sagging of the waveguide may arise because of elastic deformation of the Waveguide, or because a creep effect may occur in the material of which the waveguide is made, and may give rise to a more permanent deformation.

It is an object of the invention to reduce the amount of sagging of the waveguide.

According to one aspect of the invention, there is provided a waveguide system in which a waveguide is substantially supported by at least partial immersion in a fluid so that the buoyancy provided by the fluid is substantially equal and opposite to the gravitational force on the waveguide.

If the waveguide is totally immersed in the fluid it is desirable that the density of the fluid should be approximately equal to the mean density of the waveguide. Alternatively, if the waveguide is level over a complete section of a combination of two immiscible fluids can be used, of which one is of greater density than the mean density of the waveguide, and the other is of lesser density than the mean density of the waveguide.

The waveguide and the fluid may be contained in a channel or duct, which may be provided with expansion ports connected to tanks, in order to accommo date the thermal expansion and contraction of the fluid.

The fluid should have a low vapour pressure, so that if any fluid leaks into the inside of the waveguide, it remains in liquid form on the wall of the waveguide and so does not affect waves in the TE mode transmitted by the waveguide. If the fluid were to vaporize, the ab sorption bands of the fluid vapour spectrum would possibly have to be considered in relation to the frequency distribution of the transmitted waves.

In order to provide the most suitable degree of buoyancy, a suitable fluid must be selected. High vacuum oil, of density about 0.85 gm/cc may be used in conjunction with glass fibre waveguidesfFor steel waveguides, glycerine, of density about 1.26 gm/cc is perhaps more suitable. If the mean density of the waveguide is high, a jacket of low density (which, for example, may be made from a rigid cellular foam material) can be used to cover the waveguide and thereby reduce the mean density of the whole so that a suitable fluid may be selected.

In waveguides as constructed at present, at the junction of two adjacent waveguide section the wall thickness increases, and the mean density of the waveguide therefore increases at these junction points. The support provided for the waveguide cannot therefore be entirely uniform along the length of waveguides of this type. Accordingly the support may be made neutral or the waveguide section between the joints, so that at the joints the waveguide weight is slightly greater than the upthrust, and the waveguide therefore tends to rest lightly on the bottom of the duct or channel at these points. Alternatively, the support may be made neutral over the whole length of waveguide, including the joints, so that the sections between joints have a slight resultant upthrust. If this procedure is adopted, the force on the waveguide tends to be downwards at the joints, where the mean density is larger, and upwards between the joints. A periodic stress is therefore introduced along the length of the waveguide, and this may lead to deformation of the. waveguide with the consequent increase in the attenuation. The former procedure is therefore preferred. There is a further advantage in the former procedure, since the waveguide does not take up its own alignment, but tends to rest at the bottom of the duct or channel which is aligned to obtain a high degree of straightness, and moreover is to follow a definite path.

It is, however, possible to use waveguide sections which are constructed, for example with a spigot, in such a way that at the junction of two adjacent sections, the wall thickness of the guide is not enlarged. Thus the mean density of the waveguide is the same at all points along its length and the waveguide can be maintained in neutral support without any periodic stresses occurring along its length.

The invention also provides a method of inserting a waveguide into a duct including the steps of securing a sealing member to a leading end of the waveguide, which member forms a seal with the walls of the duct, and pumping fluid into the space between the waveguide and the duct walls, so that the pressure on the sealing member tends to draw the leading end of the waveguide into the duct. The fluid used to draw the waveguide into the duct may be that intended for use to support the waveguide in the duct, or it may be a different fluid, for example, air. This method of installation has the advantage of decreasing the wear on the waveguide support points during installation. The duct may be filled with the fluid used to support the waveguide after the waveguide has been inserted by evacuating, the space between the waveguide and the duct, after which the fluid may be injected. The evacuation of the spacejis to avoid the formation of air pockets in the duct.

- The invention will now be described by way of example with reference to the accompanying drawings, of which:

FIG. 1 represents a waveguide supported in a duct according to one form of the invention;

FIG. 2 represents a first method of filling the duct with fluid;

FIG. 3 represents a second method of filling with fluid;

FIG. 4 represents a form of waveguide suitable for 'use in'the method shown in FIG. 3; and

FIG. 5 represents another form of waveguide which can be supported in a duct in a manner according to the invention.

Referring first to FIG. 1, there is shown a waveguide 1, containing nitrogen and supported inside an underground duct 2. The waveguide rests on supports 3 disposed along its length, which may conveniently coincide with junctions of adjacent waveguide sections. The space between the waveguide l and the wall of the duct is filled with a fluid 4. Along the duct, there is provided the duct a riser tube 5 leading from inside the duct to a header tank 6, which maintains a reserve of fluid to takeaccount of volume changes due to thermal expansion, al though temperature changes underground will probably be small-The density of'the fluid is so chosen that the weight of the waveguide between any adjacent two of the supports3 is substantially equal to the weight of fluid-it displaces, this condition being referred'to herein as neutral equilibrium. The supports 3 'are of'gr eater density than the fluid so that the waveguide rests on the supports 3-at the bottom of the duct 2, with the waveguide'sect'ions between the supports 3 in a substantially unstressed condition.

In FIG. 2 there isshown a waveguide 1 having been inserted into a duct 2, before the introduction of the fluid into the duct, illustratinga method of filling the duct with fluid. The duct 2' is connected at one end, through a'valve 7 to a'vacuum pump 8, which is used to create a partial vacuum in the space between the waveguide 1 and the walls of the duct 2'. A reservoir 10 of fluid is connected to the other end of the duct 2 through a valve 9 which is then opened so that fluid contained in a reservoir 10 can be fed into the partially evacuated space. A pump 11 is provided in the connec tion of the reservoir 10 to the duct to assist the flow 'offluid, although it would be drawn into the duct by the suction.

This procedure ensures that no pockets of air are left in the duct 2, such air pockets being quite likely to form if the duct is not linear. It is possible to employ this method of filling the duct without the pump 11.

FIG. 3 shows a second method of filling the duct with fluid in which the waveguide is fed into the duct simultaneously with the fluid. An annular gasket 12 is secured to the leading end 13 of the waveguide l, which is maintained in the centre of the duct 2 by means of wheels 14 mounted on a framework 15 which supports the gasket 12. At the end of the duct where the waveguide enters, is mounted a fluid reservoir 10 connected to the duct through a pump 11. This end of the duct is covered by an annular seal 16 which prevents the escape'of fluid from the annular space around the waveguide. The seal 16 may include a bleeding valve (not shown) for removing air from the duct. The fluid is pumped from the reservoir 10 under pressure from the pump 11, into the duct surrounding the waveguide, and

forces the leading end 13 of the waveguide into the duct'by the effect of the fluid pressure on the annular gasket 12. Evacuation of the duct is not necessary to eliminate air-pockets because the space around the waveguide is always filled with fluid. The insertion of the waveguide into the duct is also achieved in a manner which reduces the loads on the waveguide relative to those imposed by other methods, since rubbing occurs during the insertion process only where the seals brush against the duct walls and'waveguide. The waveguide itself is floated into the duct in the fluid and is not pressed against the duct wall. This method can, therefore, be used for the insertion of long sections of wave guide, but the external diameter of the waveguide has to be constant if this method is to be used, since the waveguide must pass through the seall6. A more complex sealing system, substituted for the seal 16 could alternatively be provided to allow the passage of waveguides with laterally projecting joints between sections.

Instead of using the fluid used for supporting the waveguide'to draw it into the duct, another fluid, such as, for example, air, may be used and then'the support fluid can be introduced by the method described above with reference to FIG. 2.

One example ofa type of waveguide joint which may be used to join waveguide sections to be fed into the duct using the method of FIG. 3, is shown in FIG. 4. In this Figure, a joint between two adjacent sections of waveguide is shown, the joint having a spigot 17, and a socket 18. The outer diameter of the waveguide is constant substantially throughout its length. A waveguide joined in this way is of constant mass per unit length and could be arranged to be in neutral equilibrium in the fluid inside the duct, so that it could take up a stress-free position, which would thus reduce the effect of small deviations from linearity of the duct.

' A signal propagated along a waveguide inevitably is subject to some attenuation due to, for example losses and mismatches, but it cansuffer additional attenuation dependent on the frequency of the signal, which is attributable to the sag of the waveguide when supported in a duct in a conventional way by means of spaced support members. This additional attenuation is particularly significant if the waveguide is of the dielectric lined type and the support members are regularly spaced, but is important in waveguides of any type. The floatation method of waveguide support described above is expected to eliminate virtually completely the attenuation due to the sagging of the waveguide between the supporting members, and also reduce that due to departure from linearity of the duct itself.

A further reduction in the attenuation due to the waveguide could also be expected because of a decrease in the amount of creep under gravitational stress that would take place in the waveguide. Moreover, the fluid surrounding the waveguide provides a gas-tight seal, thus preventing, in particular, the penetration of oxygen and water vapour into the waveguide so that the attenuation resulting from the presence of these A two gases in the waveguide would be avoided. There is also no need for a duct pressurisation system and consequently the nitrogen pressure in the waveguide can be relatively low. Other advantages may accrue from the sealing of the waveguide by the fluid, such as the reduction of corrosion of the waveguide, and indeed also of the duct. Gas leakage from the waveguide can be monitored by observing the amount of fluid dis placed from the duct.

If the installation method described above with refer ence to FIG. 3 is employed, relatively long lengths of waveguide can be inserted without undue wear on the supporting members, so that thespacing of. the man holes for inserting the waveguide can be bigger and consequently fewer manholes are needed.

As a result of the buoyancy provided by the fluid straightness deviations in the horizontal plane arising from friction between the waveguide and the duct are largely eliminated, stress on the waveguide due to ten-- a particular application, and a suitable fluid with an equally high density may not be available for floating the waveguide. In order to overcome this problem, there may be utilised the construction indicated in FIG. 5, in which the wall of the waveguide 1 is jacketed by a covering layer 22 of a low density material, such as a rigid closed cell foam of plastics material, so as to reduce the mean density of the waveguide to that of a suitable fluid for supporting the waveguide.

Because the frictional force between the waveguide and the duct wall is low in waveguide support systems according to the invention, deviation from straightness in the horizontal plane due to lateral frictional forces is largely eliminated. Similarly, becuase of thelack of friction, when the waveguide is put under tension to take account of thermal expansion, the tension is substantially constant along the length of the waveguide, instead of having a minimum value at the centre due to longitudinal frictional effects expansion/compression sections can, of course, be used in the waveguide line to take account of thermal variations.

The invention allows support members for the waveguide to be distributed in such a way that the influences of deviation from straightness of the duct can be reduced, there being no necessity to have the support members close together in order to reduce sagging due to the weight of the waveguide.

Where there is a bend in the duct so that a corner can be turned, in conventional systems there is a tendency for the waveguide to ride up and down the side of the duct and thereby introduce spurious bends in the waveguide due to the varying amount of thermal expansion of the guide and the gravitational force on the waveguide. Since, in a system according to the invention, this force is counteracted, the tendency of the waveguide to ride up and down the side of the duct is removed.

The leakage rate of gas from the waveguide may be monitored by observing the amount of fluid forced out of the duct.

The cost of microwave transmission line using a waveguide supported according to the invention may be lower than that of one supported in the conventional way, since less amplification of the signal is required because of the reduced attenuation, the duct does not need to be pressurised, so that lower nitrogen pressures may be used in the waveguide and the gas sealing systems for the waveguide simplified, and moreover strengthening and stiffening members do not need to be provided for the waveguide.

I claim:

1. A waveguide system for long distance signal transmission systems including a waveguide and a duct containing a liquid, wherein the waveguide is substantially supported within the duct by at least partial immersion in the liquid so that the buoyancy provided by the liquid is substantially equal and opposite to the gravitational force on the waveguide, whereby attenuation in the waveguide due to sagging is reduced.

2. A system according to claim 1 wherein a jacket of low density material is attached to the waveguide.

3. A system according to claim 1 wherein the fluid has a low vapour pressure.

4. A system according to claim 1 wherein the waveguide is wholly immersed in fluid.

5. A system according to claim 4 wherein the fluid includes two immiscible fluids of differing densities, one of greater density and one of lesser density than the mean density of the waveguide.

6. A system according to claim 4 wherein the waveguide has supports spaced along its length, the mean density of the waveguide between the supports being substantially equal to that of the fluid, and the mean density of the waveguide at the supports being greater than that of the fluid, so that the supports tend to rest on the bottom of a means containing the fluid.

7. A system according to claim 1 wherein the duct is closed at the top and is mounted under the ground.

8. A system according to claim 7 wherein the duct has a plurality of reservoirs at spaced points, thereby to accommodate thermal expansion and contraction of the fluid. 

1. A waveguide system for long distance signal transmission systems including a waveguide and a duct containing a liquid, wherein the waveguide is substantially supported within the duct by at least partial immersion in the liquid so that the buoyancy provided by the liquid is substantially equal and opposite to the gravitational force on the waveguide, whereby attenuation in the waveguide due to sagging is reduced.
 2. A system according to claim 1 wherein a jacket of low density material is attached to the waveguide.
 3. A system according to claim 1 wherein the fluid has a low vapour pressure.
 4. A system according to claim 1 wherein the waveguide is wholly immersed in fluid.
 5. A system according to claim 4 wherein the fluid includes two immiscible fluids of differing densities, one of greater density and one of lesser density than the mean density of the waveguide.
 6. A system according to claim 4 wherein the waveguide has supports spaced along its length, the mean density of the waveguide between the supports being substantially equal to that of the fluid, and the mean density of the waveguide at the supports being greater than that of the fluid, so that the supports tend to rest on the bottom of a means containing the fluid.
 7. A system according to claim 1 wherein the duct is closed at the top and is mounted under the ground.
 8. A system according to claim 7 wherein the duct has a plurality of reservoirs at spaced points, thereby to accommodate thermal expansion and contraction of the fluid. 